The invention relates to a method for the control of disturbances in the preparation of hydrocarbon hydroperoxides by oxidation of hydrocarbons with molecular oxygen. More particularly, the present invention relates to control of runaway decomposition of hydroperoxides in such preparations by a process which comprises steps for sensing the onset of runaway and, in response thereto, adding to the oxidation reaction mixture in a substantially instantaneous manner of small quantity of a base.
It is well known in the chemical arts that hydroperoxides of hydrocarbons, for example, isobutane, cyclohexane, cumene, and ethylbenzene, can be prepared by contacting the hydrocarbon with molecular oxygen at an elevated temperature. The resulting hydroperoxides have recognized value, for instance, as catalysts, as initiators for free radical-type reactions, and as starting materials or intermediates in the production of chemicals such as oxirane compounds and other hydroperoxides.
A feature of hydrocarbon hydroperoxide manufacture that is the subject of substantial concern to the art is decomposition of the hydroperoxide in the oxidation reaction mixture, which is known to occur in response to any one or more of a great number of stimuli. Under normal operation, hydrocarbon hydroperoxide preparation is accomplished in such a manner that the decomposition of the product hydroperoxide which occurs is limited to an acceptably low level. For this purpose, precautions are typically taken to insure, for instance, that reaction mixture temperature is maintained within a specified range and that hydrocarbon and oxygen reactants are essentially free of impurities known to cause the decomposition. It is also common practice to minimize this low-level decomposition by limiting residence time of the reaction, e.g., through the addition of reaction promoters which decrease the reaction's induction period and increase its rate or through a termination of the reaction at a relatively low conversion of the hydrocarbon to hydroperoxide. Nevertheless, because of misoperation, equipment failure, or the like, there occur instances of abnormal, accelerated decomposition in the reaction mixture which cannot readily be managed. This decomposition is accompanied by the formation of decomposition by-products, e.g., alcohols, organic acids, aldehydes, and ketones, and by the release of heat and a consequential increase in temperature in the reaction mixture. In such circumstances the hydroperoxide decomposition is said to be autocatalytic, since the by-product chemicals and increased temperature both promote further accelerated decomposition.
Instances of uncontrolled, accelerated decomposition of the hydroperoxidation reaction mixture are herein termed "runaway". If allowed to proceed unchecked, runaway will, of course, result in the loss of valuable hydroperoxide product. Of substantially greater concern, however, is the danger that the runaway will accelerate to explosion of the hydrocarbon hydroperoxide/oxygen/hydrocarbon mixture. In some cases, potentially explosive conditions are encountered with only moderate increases in temperature, e.g., 10.degree. to 20.degree. C., above that of normal operation.
There are available in the art a number of teachings directed to the management of runaway reactions in hydrocarbon hydroperoxide preparation. According to one approach to the problem, there is provided in the oxidation reactor excess heat exchange capacity which can be put into use to compensate for the heat release associated with accelerated decomposition of the hydroperoxide product. However, the effectiveness of this approach is limited, and the expense involved restricts its practical application. Another alternative for management of runaway, disclosed in Netherlands patent specification No. 7511955, relates to the introduction of substantial quantities of water into the oxidation reactor whenever runaway is encountered to dilute and cool the reaction mixture.
With respect to aspects of the present invention which relate to the addition of a base to a runaway hydroperoxide reaction mixture, it is to be noted that bases have conventionally been utilized for a variety of purposes in hydrocarbon oxidation. For instance, U.S. Pat. No. 2,632,772 teaches addition of an alkali to the reaction mixture. It is said that apart from preventing the corrosion of iron reaction vessels, the effect of this alkali addition is that the maximum rate of oxidation is higher and is obtained with a shorter induction period, and that the peroxide concentration in the reaction mixture may be greater than in the absence of alkali. On the subject of the management of runaway in the reaction, the patent teaches only that temperature should be reduced as the reaction proceeds, and that residence time of the mixture in the oxidation reactor should be limited to avoid accelerated hydroperoxide decomposition. Such measures may be undesirable, however, because of adverse influence on oxidation reaction conversion, selectivity, and rate. Great Britain Pat. No. 713,138 teaches a process for the oxidation of cumene in the presence of secondary or tertiary amine reaction promoters. The amine is added to the reaction mixture in a quantity based upon the concentration in the hydrocarbon feed of certain impurities which bind free radicals, interrupting the chain reaction mechanism by which the oxidation reaction proceeds. The patent teaches that the presence of the amines counteracts the effect of the impurities and results in a shorter reaction induction period. It is also suggested that the presence of the amines during the oxidation reaction decreases the production of color-forming reaction by-products as the result of the low-level hydroperoxide decomposition normally observed in such oxidation reactions. There is no mention in this patent of methods for bringing runaway decomposition under control.